The use of static relays in the field of power transmission and distribution has increased rapidly in recent years due to the absence of moving mechanical parts resulting in a long and reliable operating life. Typical of such static relays is that described in U.S. Pat. No. 3,319,127 assigned to the assignee of the present invention.
Circuits of the foregoing type monitor the a.c. current flowing through a circuit and apply the monitored current to a timing circuit which generates a tripping signal in accordance with an inverse time-current relationship depending upon the magnitude of the overload condition being sensed. Whenever a sufficiently severe overload condition occurs, the timing circuit generates a tripping signal which trips a protective device such as a circuit breaker.